The present invention relates to a method and composition for the treatment of wood and wood products and more particularly the treatment of wood with an insecticide.
In particular environments terrestrial borers such as termites have been known to readily damage timber products. Therefore, timber products are often impregnated with insecticides, preferably pyrethroids so as to control borer, termite, and other insecticidal damage.
An insecticide such as a pyrethroid may be impregnated into timber in the presence of hydrocarbon solvents, aqueous solutions, emulsions and microemulsions, and dispersions. However, many refractory timbers are resistant to impregnation due to the high surface tensions of the solvents that inhibit flow into the microporous structure of timber, even if external pressure is applied. Therefore, the existing methods of impregnating insecticides such as pyrethroids into timber in the presence of any of the previously mentioned carriers does not give satisfactory penetration of preservatives into the heartwood of many softwoods and hardwoods, and for some species the sapwood as well.
Similarly, many wood composites cannot be treated in final form due to barriers to liquid flow resulting from the presence of glue and refractory heartwood or sapwood. The result is often a thin envelope treatment of an insecticide such as a pyrethroid that termites in particular can readily penetrate, allowing them access to an untreated core.
Supercritical fluids have been used for the deposition of various materials onto or into a substrate, however, there have been relatively few publications disclosing the use of supercritical fluids in the treatment of wood and wood products.
Ito et al. in Japanese Pat No. 59-101311 provides the first suggestion of using supercritical fluid to treat wood, however, the disclosure provides few details and no examples to show how the method would work in practice. Subsequent studies suggest that many of the stated objectives were not achieved (Morrell et al., American Wood-Preservers"" Association 1997, 93, April, pp. 367-386).
Sunol in U.S. Pat. No. 4,992,308 discloses a method and process using a supercritical fluid, with or without the aid of entrainers to solubilize a monomer, monomer mixture or polymer, to carry the supercritical solvent mixture thereby created into the wood matrix, to remove extractives from the wood, to precipitate the monomer or polymer within the wood, and to polymerise the monomer in situ in the wood. Polymerisable monomers included methyl methacrylate (MMA), dialkylphthalate, ethyl acrylate, styrene, and many others. Other patents are also referred to in the Sunol patent. These showed examples of treatments for wood and other porous materials, but all of the disclosures dealt with monomers and polymers. These examples did not include insecticides, as the applications of the inventions disclosed in the various patents related specifically to the improvement of mechanical properties of wood such as hardness and bending strength rather than durability against biological agents.
Kayihan in U.S. Pat. No. 5,094,892 discloses a method that can be used in the impregnation of lumber or structural timbers with a preservative, however, the specification did not disclose an example of impregnation with a preservative, rather only with a dye. Also, the Kayihan patent only applies to a chemical that is insufficiently soluble in supercritical carbon dioxide.
Morrell et al., Canadian Wood Preservers Association, 1993, No. 14, pages 6 to 25 discussed the potential of using supercritical fluid processes to treat wood, but all discussion and examples related to fungicides.
It would appear that no prior study has examined the feasibility of impregnating wood or wood products with insecticides including the pyrethroids. Nor have they determined whether a co-solvent is required, or provided examples of successful wood treatment processes.
It is therefore an object of the invention to provide a method and composition for the treatment of wood with an insecticide using a supercritical fluid.
According to a first aspect of the invention there is provided a method of treating wood with an insecticide, including impregnating intact wood with an effective amount of an insecticide in the presence of a supercritical fluid under conditions that enable sufficient insecticide to dissolve in the supercritical fluid.
In a preferred embodiment of the first aspect of the invention, the method includes treating wood with an insecticide, the method including the steps of:
passing a supercritical fluid through a bed of insecticide to form a solution of insecticide in the supercritical fluid,
impregnating wood with the solution at a temperature and pressure in excess of supercritical levels and decreasing the temperature and pressure below supercritical levels to precipitate the insecticide within the impregnated wood.
It is preferred that the pyrethroid is any one of permethrin, cypermethrin or deltamethrin, though other pyrethoids may be utilised in the wood treatment process. The insecticides may also be impregnated in conjunction with fungicides.
Supercritical fluid ceases to be a liquid and behaves like a gas, although retaining the solvent properties of a liquid. Though it is preferred that the supercritical fluid utilised in the wood treatment process is carbon dioxide, other supercritical fluids can be utilised.
Carbon dioxide at a temperature greater than its critical temperature of 31.1xc2x0 C. and a critical pressure of 72.9 atm. or 7.39 MPa is widely used as a supercritical fluid. This is due essentially to the fact that supercritical carbon dioxide has a relatively low critical pressure and temperature, low cost, is readily available in large quantities, and is non-toxic, odourless, and non-flammable. Its surface tension becomes zero with a viscosity of about 320 xcexcP (at 40xc2x0 C. and 75.1 atm.). It has very good solvent characteristics and is environmentally sound without having any pollution and waste disposal problems.
In determining the suitability of supercritical fluid technology as a carrier for various chemicals, solubility is probably the most important thermophysical property that must be acquired and modelled for an efficient design of any industrial scale impregnator (Ashraf-Khorassani et al., 1997; Madras et al., 1993). Both temperature and pressure can affect the solubility of a solute in a supercritical fluid (Schmitt and Reid, 1986). Raising the temperature can dramatically increase the vapour pressure of the solute and consequently its solubility (Miller and Hawthorne, 1994). Hence, the solubility must be determined as a function not only of pressure, but also of temperature (Hampson, 1996).
A co-solvent may be used to increase the solubility of a particular insecticide in supercritical fluid. Suitable co-solvents include alcohols, but methanol is preferred.
According to a second aspect of the invention, there is provided a composition for treating wood, the composition including an effective amount of an insecticide dissolved in a supercritical fluid.
Preferably the supercritical fluid is carbon dioxide and the insecticide is a pyrethroid. It is preferred that the pyrethroid is any one of permethrin, cypermethrin or deltamethrin, though other pyrethoids may be utilised.
Preferably the composition includes a co-solvent. The co-solvent may be an alcohol and is preferably methanol.